1. Field of the Invention
The present invention relates to the gasification of carbonaceous solids to produce a gas comprising H.sub.2, C0 and CH.sub.4. More particularly, the invention relates to an improved C0.sub.2 acceptor process wherein the C0.sub.2 acceptor and the carbonaceous solids are contacted in countercurrent plug-like flow.
2. Prior Art
As a result of the dwindling supplies of petroleum and natural gas, extensive research efforts have been directed towards the conversion of coal into suitable gas or liquid fuels. In comparison to known petroleum and natural gas reserves, coal supplies are abundant and the United States is fortunate to have approximately one-third of the world's known coal reserve. Coal may be gasified by a number of processes to produce combustible gases. These gases may generally be upgraded by the familiar shift conversion to produce a high BTU content gas of pipeline quality, or used directly as an industrial source of low to medium BTU content gas or converted into liquid fuels by a Fischer-Tropsch type synthesis.
Of the many coal gasification processes under investigation for commercial purposes in the United States, the promising and unique CO.sub.2 acceptor process merits serious consideration. The mineral dolomite, a calcium-magnesium carbonate, serves a unique role in the process and is the basis for the process name. If dolomite is calcined at 1800.degree.-1900.degree. F., CO.sub.2 is released and the dolomite is transformed from a carbonate to an oxide state. In the oxide state the dolomite will chemically combine with gaseous CO.sub.2 and thus afford means for the removal of same from a process stream. The "acceptance" of CO.sub.2 by the oxide form dolomite is exothermic and the heat of reaction may be used to advantage in the process.
In the basic CO.sub.2 acceptor process, steam is reacted with crushed lignite in a fluidized bed gasifier at a temperature of approximately 1500.degree. F. to produce CH.sub.4, CO, CO.sub.2, and H.sub.2. Hydrocarbons, including tars, above ethane or propane are cracked under the severe gasification conditions to produce CH.sub.4 and coke, and, thus, do not appear in the product. Heat for the endothermic gasification process is provided by showering calcined dolomite at a temperature of approximately 1850.degree. F. through the fluidized bed of lignite. Sensible heat transfer occurs in the fluidized bed as the dolomite cools from 1850.degree. F. to 1500.degree. F. and additional heat is generated by the exothermic reaction of the oxide form dolomite with CO.sub.2 to produce the carbonate form dolomite. The spent acceptor, or carbonate form dolomite, is withdrawn from the gasifier and calcined in a separate vessel to produce the oxide form dolomite for recycle to the gasifier. Thus, the dolomite serves the two important functions of providing heat for the gasification reactions and removing CO.sub.2 from the product gas.
While attractive from a theoretical standpoint, the existing CO.sub.2 acceptor gasifiers are limited by solids and gas throughout. The gasification is carried out in a fluidized bed of lignite with steam and recycle synthesis gas passing upwardly through the bed as a fluidization medium. The fluidization gas velocity is, therefore, restricted to a range between the minimum fluidization velocity and the terminal velocity of the lignite particles in the bed, and a value of about 1 foot per second appears to be typical. For a fixed gas composition, the amount of carbon gasified (lb/hr/ft.sup.2) is dependent only upon the gas velocity. In the article "CO.sub.2 Acceptor Process" appearing in the proceedings of the Sixth Pipeline Gas Symposium, Chicago, 1974, by C. Fink et al, the published information indicates a typical carbon gasification rate of approximately 140 lb/hr/ft.sup.2. This low gasification rate dictates that the reactor will have a low length to diameter ratio, tending to make it very expensive on a commercial scale.
It is therefore an object of this invention to provide a unique gasification process for a CO.sub.2 acceptor system which will result in a much greater throughput capacity and a corollary reduced gasifier capital expense, while retaining the salient advantages of the basic process.